The present invention relates to a hydrogen purifying apparatus and method for use in fuel cells such as solid polymer electrolyte fuel cell. More specifically, the present invention relates to a hydrogen purifying apparatus and method for reducing a concentration of carbon monoxide in a modified gas containing gaseous hydrogen and carbon monoxide.
As the fuel gas for use in fuel cells, a modified gas is used which can be obtained by reforming a material, such as hydrocarbon including natural gas, alcohol including methanol, or naphtha, with a water vapor. Such modified gas normally contains byproducts including carbon monoxide and carbon dioxide in addition to hydrogen gas.
Fuel cells which can operate at high temperatures, such as molten carbonate fuel cell, can even utilize carbon monoxide as a fuel. However, for fuel cells with lower operating temperatures, such as phosphoric acid fuel cell and solid polymer electrolyte fuel cell, the presence of a high concentration of carbon monoxide in the hydrogen gas is poisonous to any platinum group metallic catalyst which is utilized by an electrode of the cell. As a result, such cell can not exert satisfactory electric power generating performance. For the solid polymer electrolyte fuel cell in particular, the electrode catalyst is poisoned with carbon monoxide in a relatively short time even if the concentration of carbon monoxide in the fuel gas is as low as 50 ppm or so, producing rapid impairment of the electric power generating performance of the cell.
Therefore, carbon monoxide is removed by oxidizing it using a platinum group metallic catalyst after reducing the concentration of carbon monoxide in the fuel gas with a carbon monoxide metamorphic catalyst.
An example of the method for removing carbon monoxide by oxidation is to oxidize only carbon monoxide selectively at low temperature using a catalyst carrying on the carrier alumina a known activator platinum or rhodium thereby removing the carbon monoxide (see Japanese Laid-Open Patent Publication No. Hei 5-201702, for example).
As an alternative, there is a method which provides an oxidation catalyst of carbon monoxide on the flow route of the fuel gas toward the fuel cell and then introduces open air in order to supply sufficient amounts of oxygen (oxidant) to the fuel gas thereby effectively oxidizing and removing carbon monoxide (see Japanese Laid-Open Patent Publication No. Hei 9-504901, for example).
According to the methods, the concentration of carbon monoxide in the fuel gas can be reduced to as low as 10 ppm or so which is lower than the poisonous concentration to the electrode catalyst.
However, under practical use conditions, since the concentration of carbon monoxide in the fuel gas changes as the amount of fuel gas supplied to the fuel cell changes, it is necessary to control the supplying amount of open air as appropriate. However, the oxidation reaction of carbon monoxide in the presence of oxidation catalyst accompanies heat generation and thus alters the temperature of the electrode catalyst when the supplying amount of open air to the fuel gas is varied. There is a problem in the prior art methods that when the catalyst temperature is altered and reaches outside the optimal temperature range of the catalyst activity, oxidation and removal of carbon monoxide becomes unsatisfactory.
Another problem is that excess supply of open air increases the amount of heat generated by the oxidation catalyst and elevates the temperature of the catalyst. Heat generation is concentrated particularly at the catalyst close to the side into which the fuel gas is introduced, producing a high temperature around there in a short time. Since hydrogen is more reactive to the catalyst than carbon monoxide, the oxygen supplied as an oxidant is mostly consumed for oxidizing hydrogen rather than carbon monoxide, if the catalyst has a high temperature. As a result, the catalyst loses the ability to selectively oxidize carbon monoxide.
Under such circumstances, it is essential in the hydrogen purifying apparatus to control the catalyst temperature in a range at which carbon monoxide readily reacts with the catalyst but hydrogen does not. In other words, reduction in the change, particularly elevation of catalyst temperature is required.
The most efficient temperature for oxidizing carbon monoxide is a critical low temperature at which carbon monoxide can react with the catalyst. However, control of the temperature of the oxidation catalyst which selectively oxidizes carbon monoxide by regulating the amount of fuel gas to be supplied to the fuel cell or by cooling the catalyst eventually excretes drastic amounts of carbon monoxide upon only a slight decrease of the temperature. Therefore, the prior art methods required control of the temperature within a range of several to several tens degrees centigrade higher than the critical low temperature, in consideration of the flow rate of the fuel gas and possible changes in the catalyst temperature. As such, the conventional methods have met difficulties considerably in achieving selective and efficient oxidation of carbon monoxide.
Furthermore, the oxygen supplied is constantly consumed for oxidizing hydrogen gas as the fuel while being consumed for oxidizing carbon monoxide. This means that there is a need to reduce the supplying amount of open air to a minimum. However, if the catalyst is elevated in temperature when the oxygen amount in hydrogen gas is insufficient due to down-regulated amounts of open air to be supplied, production of carbon monoxide proceeds due to reaction equilibrium between the carbon dioxide and hydrogen. Therefore, the air to be supplied must be controlled exactly and precisely to a right amount. In order to satisfy the above requirement, the conventional hydrogen purifying apparatus meets a problem that it is inevitably complicated in structure.
In view of the above-mentioned various facts, the object of the present invention is to provide a hydrogen purifying apparatus in which the catalyst for selectively oxidizing carbon monoxide can exert the ability sufficiently and the concentration of carbon monoxide in the fuel gas (hydrogen gas in this case) can be reduced constantly and stably even when the use conditions of the apparatus, such as temperature, amounts of open air to be supplied and amounts of carbon monoxide to be treated are varied.